1. Field of the Invention
This invention relates to apparatus for, and a method of, installing a transmission line such as an optical fibre telecommunications line.
2. Related Art
In the United Kingdom, the telecommunications network includes a trunk network which is substantially completely constituted by optical fibre, and a local access network which is substantially completely constituted by copper pairs. Flexibility in the copper access network is provided at two points en route to the customer; firstly, at street-side cabinets serving up to 600 lines; and secondly, at distribution points serving around 10-15 lines. Eventually, it is expected that the entire network, including the access network, will be constituted by fibre.
The ultimate goal is a fixed, resilient, transparent telecommunications infrastructure for the optical access network, with capacity for all foreseeable service requirements. One way of achieving this would be to create a fully-managed fibre network in the form of a thin, widespread overlay for the whole access topography as this would exploit the existing valuable access network infrastructure. Such a network could be equipped as needs arise, and thereby could result in capital expenditure savings, since the major part of the investment will be the provision of terminal equipment on a `just in time` basis. It should also enable the rapid provision of extra lines to new or existing customers, and flexible provision or reconfiguration of telephony services.
In order to be completely future proof, the network should be single mode optical fibre, with no bandwidth limiting active electronics within the infrastructure. Consequently, only passive optical networks (PONs) which can offer this total transparency and complete freedom for upgrade, should be considered.
The most common passive optical network is the simplex single star, with point-to-point fibre for each transmit and receive path, from the exchange head end (HE) to the customer network terminating equipment (NTE). This network design has been used throughout the world and meets all the access criteria. It involves high fibre count cables, and unique electro-optic provision at HE and NTE for each customer. The resulting inherent cost can only be justified for large business users, who generally also require the security of diverse routing, which increases the cost still further.
The advent of optical splitters and wavelength-flattened devices has enabled the concept of the PON to be taken one step further. These passive components allow the power transmitted from a single transmitter to be distributed amongst several customers, thereby reducing and sharing the capital investment.
The use of splitter based PON architecture thus reduces the cost of fibre deployment in the access network. When compared with point-to-point fibre, savings will result from:
(i) reducing the number of fibres at the exchange and in the network; PA1 (ii) reducing the amount of terminal equipment at the exchange; PA1 (iii) sharing the cost of equipment amongst a number of customers; PA1 (iv) providing a thin, widespread, low cost, fibre infrastructure; and PA1 (v) providing a high degree of flexibility, and allowing `just in-time` equipment and service provision.
Additionally, PON architecture can be tailored to suit the existing infrastructure resources (duct and other civil works).
It will be apparent that upgrading the entire UK access network from copper to fibre will involve a major capital investment program. It is important, therefore, to minimize costs wherever possible. The specifications of our published International patent applications WO95/07475, WO95/07476, WO95/07477. WO95/07478 & WO95/107486 describe a fibre management system which aims to reduce the cost of providing fibre from local exchanges to the network nodes (equivalent to the distribution points of the copper access network) nearest the customers. The specifications of our published International patent applications GB95/00449 and GB95/00450 describe a way of minimizing the cost of getting fibre into a customer's premises via a customer lead in (CLI) provided in an external wall of the premises. The present invention is concerned with minimizing the cost of getting fibre from just outside a customer's premises to the nearest network node.
German,patent number DE3826513 discloses a method for laying a transmission line under the ground alternative to the established method of digging a trench in the ground along the intended route of the line, laying the line into the trench and then backfilling to bury the line. One embodiment of the apparatus has a pneumatically driven tunnelling head whose route is guided by a C-shaped guide attachment mounted on the end of an arm extending radially from the tunnelling head.
Published UK patent application number 2085670 discloses a device for loosening the earth around a previously buried cable. The device has a main body comprising inner and outer cylindrical sections, which are moved alternatively, by hydraulic means, relative to one another, such that the apparatus moves along the previously buried cable with a "shinning" movement. As the apparatus moves along the cable, water is supplied to the apparatus and is ejected through nozzles at the front of the apparatus to loosen the earth around the cable in front of the apparatus.